Induction plasma torch



Nov. 24, 1970 A. J. BURGGR AAF INDUCTION PLASMA TORCH Filed Dec. 8, 1967 IN VENTOR ANTHONIE J .BURGGRAAF AGENT United States Patent US. Cl. 131 4 Claims ABSTRACT OF THE DISCLOSURE An induction plasma torch for heating finely divided material. A cylindrical casing is surrounded by an induction coil and sealed at both ends, one end having apertures for introducing a supply gas, and a gas containing a mixture of finely divided material. The resultant plasma gas is removed through exhaust apertures in this same end.

The invention relates to an induction plasma torch for heating finely divided material. The torch is provided with a cylindrical casing surrounded by an induction coil, the casing has a seal at one end through which are provided at least one supply aperture for a stream of gas, which travels along the inner wall of the casing. A second supply aperture is provided for a stream of a mixture,

of gas and finely divided materials which is axially directed into the casing. The torch is used for heating or melting pulverulent or gritty material having a high melting point such as refractory oxides or carbides of nuclear fuels. In presently used plasma torches, the gas is conducted through the casing and out on a side other than through which the gas enters the casing. A recurrent problem of the prior art torches occurs due to the relatively large quantity of gas required for the desired stability of the plasma; consequently the stay or period of suspension of the particles of the pulverulent or gritty material in the plasma is often too short to process an intended quantity of material. A plurality of factors, such as the velocity of the gas along the wall, the quantity of mixture per unit of time, the composition of said mixture and the supplied radio-frequency energy must be relatively adjusted so as to achieve the desired stability of the plasma, and the adjustment is particularly critical especially if a longer stay of the pulverulent material in the plasma is desired. If the melting behaviour of the powder in a plasma obtained by radio-frequency induction does not satisfy the expectations and if one or more adjustments must be modified to this end, one is concerned time and again with the less satisfactorily controllable stability of the discharge. When melting quartz grit or quartz powder it is particularly important to obtain a sufliciently long stay of the particles in the plasma in order that each particle is heated in such manner that not only the envelope but also the nucleus thereof is in a molten condition so as to obtain a melting bath being completely free of gas inclusions which are readily carried along with the particles in the bath in case of incomplete melting and have a disadvantageous influence on the usability of the manufactured product when the quartz is further processed.

An object therefore of this invention is to provide an induction plasma torch with which the said operating adjustments can be realized without too much critical interdependence. According to the invention the cylindrical casing is sealed in a gastight manner at one end; the other end is provided with a seal through which there are supply apertures for gas for pulverulent material mixed with gas, and, one or more exhaust apertures for conducting away the gas plasma from within the casing.

A radio frequency induction coil surrounds the casing at the gastight sealed end. The gas stream enters and moves along the wall such that at the area near the coil there is present an outermost gas stream moving towards the sealed end of the casing and an inner gas stream moving in the opposite direction and displaced more towards the centre of the casing.

The stabilization of the gas plasma discharge generally improves when less heat is dissipated. It is an advantage therefore to have a gas flow produced such that the heat therein carried along the wall by the gas stream is fed back within the discharge area rather than being dissipated through the casing walls. It has been found that with such a torch, the quantity of gas to be supplied per unit of time under otherwise identical conditions may be considerably smaller than if the gas is conducted away from the casing at the end remote from the inlet side. This reduced supply of gas renders a longer stay of the finely divided material possible which is further increased because the direction of the gas stream towards the centre of the casing is opposite to that of the stream of the particles. A still further and considerable reduction of the required quantity of gas is obtained if the gas stream along the wall is not only axially directed but has a tangential component of movement in addition to a velocity in the forward direction, with the result that the gas performs a helical movement along the wall when flowing from one end to the other.

A further advantage of the plasma torch according to the invention is that the gas reversing its direction at the sealed end of the casing and supplied along the Wall supplies heat to the molten material collected in situ so that the melting bath provided for this purpose may be larger than if heating only took place by the plasma extended in the direction of the melting bath.

The orifice of at least one exhaust duct on the same side as the end of the casing where the gas is supplied is preferably located more closely to the axis than the apertures of one or more ducts for the supply of gas only. In an efi'icient construction of the seal at the said end of the casing, said supply apertures are arranged concentrically about the axis, the orifices for a plurality of exhaust ducts being arranged concentrically at a smaller distance from the axis, the assembly of apertures and orifices being arranged concentrically about a duct in the axis for supplying a gas stream which is mixed with the pulverulent or gritty material. When the melting bath is sufiiciently large, the supply of grit may take place in a larger section through an annular aperture or a plurality of apertures around a central aperture, the latter being used in that case for conducting away the gas.

In order that the invention may be readily carried into effect, it will now be described in detail, by way of example, with reference to the accompanying diagrammatic drawing, in which further particulars relating to the invention will be dealt with.

The figure shows an induction plasma torch having a space 1 enclosed by a casing 2 within which the electric gas discharge, called plasma, takes place. The casing 2 is made of an insulating material having a high melting point and causing no considerable dielectric losses when using a radio-frequency induction field. The casing usually consists of quartz or of a double-walled quartz tube, a coolant being led through the interspace. The use of a metal casing is also possible if it is ensured that no shortcircuit currents are produced in the wall. An induction coil 3 consisting of one or few turns surrounding the casing may be connected to a radio-frequency generator of known construction which is therefore not shown. Upon energizing, the coil 3 induces a magnetic field of high frequency in the space 1 within the casing 2, and a suitable gas atmosphere for producing a plasma is created. Generally, argon is used as a filler gas for the ignition, however once the plasma is obtained other gases for example, nitrogen is usable for maintaining the plasma. During ignition the argon gas is supplied in known manner and is replaced by nitrogen gas when the tube is normally operating.

Supply ducts in transverse wall or seal 4 terminate in the space 1 within the casing and admit a gas flow which travels along the inner wall of the casing, the path of travel being indicated by dashed line 6. The gas flows along the casing wall to the opposite end of the casing, there it is deflected inwardly and then flows in the reverse direction toward the direction of entry. The plasma gas is conducted away, through exhaust apertures 7 provided in the seal 4 said apertures being located at a shorter radial distance from the centre than the supply apertures 5.

The material to be melted is supplied through tube in the centre in the seal 4 which tube has anaperture 11 for axial passage of a gas stream carrying with it the grit or powder to be melted.

The casing 2 is sealed against the passage of gas at the opposite end by a transverse wall 8 having a cup-like depression 9 in which the molten material is collected and forms a melt.

The direction of the probable gas flow as shown is obtained when the supply ducts 5 terminating through the seal 4 are to some extent placed tangentially at an angle. It has been found that with a gas flow of sufiicient intensity for stabilizing the plasma, the stream will convey the plasma away from the casing wall and thus prevent melting. The flow will follow the wall and deflect upward at the sealed end of the casing whereas with an axial supply of gas, the gas density along the wall is smaller and a stronger flow is necessary for sufficient stabilization and heat insulation. The plasma will become unstable either with too high or too low a velocity of the gas. It has been found that with an adjustment of the gas supply between 5 and L/min. it is possible to maintain a stable plasma if the inside diameter of the casing is between 70 and 95 mms. Such a plasma torch may be loaded with 18 to 25 kw. Observations proved that the gas moving along the transverse wall 8 contributes considerably to maintaining the collected, molten material in the cup-like depression 9 at its melting point and if the melting bath is not heated separately, an etfective engagement of the diameter of the melting bath is obtained in that the plasma widens on the melting surface.

Known plasma torches having their gas exhausts at the end facing the supply require approximately 30 1./min. at a loadability of 12 kw., which means that a plasma torch having the same characteristics operates considerably more efiiciently. The gas flowing back then enables an adjustment of the supplied quantity of grit or powder of the material of high melting point within not too wide limits, because their stay in the plasma is adjustable both by the intensity of the gas stream carrying the material along, and by the intensity of the stabilizing gas stream. Results are achieved in which at least 50% of the material to be melted is collected in the melting bath if the ratio of both gas streams is between 1 in 3 and 1 in 4.

The location of the exhaust apertures is not limited to the region within the ring of supply apertures.

When the supply apertures are provided at distances regularly distributed along the circumference, the exhaust apertures may be provided between the supply apertures such that the conducting away of the gas does not interfere with the supply of gas.

What is claimed is:

1. An induction plasma torch for heating finely divided material comprising a cylindrical casing, an induction coil surrounding the casing, a first seal at one end of the casing, a second seal at the other end of the casing, said first seal defining at least one gas supply aperture therein for supplying a stream of gas along the inner wall of the casing in the area of the coil, said stream being reversed at the opposite and sealed end of the casing and deflected inwardly and in the direction of entry, at least one additional aperture defined by said first seal for supplying a stream of gas contained finely divided material, the last mentioned stream being directed along the axis of the casing, and at least one exhaust aperture defined by said first seal for conducting away gas from within the casing.

2. A plasma torch as claimed in claim 1 wherein exhaust apertures defined by the first seal, are located radially closer to the axis then the gas supply apertures.

3. A plasma torch as claimed in claim 2 wherein gas supply apertures and exhaust apertures are arranged concentrically about a central aperture.

4. An induction plasma torch for heating finely divided material comprising a cylindrical casing, transverse walls sealing both ends of the casing, an induction coil surrounding the casing, a central aperture and a plurality of concentric apertures defined by one of the transverse walls, wherein a gas is supplied through the radially outermost aperture, a gas containing finely divided materials is supplied through the central aperture and the resultant plasma gas is removed through exhaust apertures positioned radially intermediate the central and outermost apertures.

References Cited UNITED STATES PATENTS 3,210,454 10/1965 Morley 313-231 X 3,296,410 1/1967 Hedger. 3,343,027 9/1967 Frohlich 219 X BERNARD A. GILHEANY, Primary Examiner R. N. ENVALL, 111., Assistant Examiner US. Cl. X.R.

1322; UNITED S'IA'IES PATENT OFFICE CERTIFICATE OF CORRECTION pa e N 3,541,625 Dated November 24, 1970 Inventor-( ANTHONIE J. BURGGRAAF It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Column 1, following "Eindhoven, Netherlands" the next line should read assignor, by mesne assignments, 1 U. S. Philips Corporation, New York, New York, a

Corporation of Delaware-- Signed and sealed this 1 7th day of August 1971 SEAL) Attest:

EDHARD M.FLETCHER,JR. WILLIAM E. SGH'UYLER, E. Attesting Officer Commissioner of Patent 

